Virtually all silver halide grains utilized in photography exhibit face centered cubic ("rock salt") crystal lattice structures. Face centered cubic lattice structures are those lattice structures that have an internal ion arrangement--a crystal lattice--akin to the arrangement of ions in standard table salt (NaCl). Each lattice structure, barring imperfections or impurities which could distort the ionic arrangement, has similar ion types (anion or cation) occupying the corners and center of each face of a cube.
Although presently utilized silver halide grains have cubic lattices, such lattices, as noted, define an internal structure. The external appearance of grains defined by cubic lattices may be cubic, but they may also take any one of a number of other morphologies.
Known morphologies for cubic lattice type silver halide grains are as described in, for example, Maskasky, "The Seven Different Kinds of Crystal Forms of Photographic Silver Halides" Journal of Imaging Science, Vol. 30, 1986, pp. 247-255. Known forms include the cube, octahedron, rhombic dodecahedron trisoctahedron, icositetrahedron, tetrahexahedron, and hexoctahedron.
In addition to describing silver halide grains according to their morphology, it is common practice in the art to describe grains by reference to their crystalline faces. Typically, Miller indices are utilized to define each face which bounds a silver halide grain. Miller indices, calculations thereof, and their manner of application are described in Crystals Perfect and Imperfect by A. Bennet, D. Hamilton, A. Maradudin, R. Miller and J. Murphy: Walker and Company, New York, 1965.
For cubic grains, the six crystal faces are usually referred to as {100} crystal faces, such reference being based upon the appropriate Miller indices. While the {100} crystal face designation is most commonly employed in connection with cubic silver halide grains, these same crystal faces are sometimes referred to as {200} crystal faces, the difference in designation resulting from a difference in the definition of the basic unit of the crystal structure.
Octahedral grains have eight identical crystal faces that are referred to as {111} crystal faces. Rhombic dodecahedral grains have twelve identical faces that are referred to as {110} crystal faces. The remaining morphologies of silver halide grains all have distinctive crystalline face arrangements. Each face can be defined by reference to the appropriate Miller indices which, in turn, can be confirmed by a combination of visual inspection and the determination of the angle formed by the intersection of adjacent crystalline faces. This method of confirming the Miller indices of a certain crystal face may also be utilized to confirm that a given face is a {100}, {111} or {110} crystal face.
Although {100} crystal faces represent the faces of a cubic grain, {111} crystal faces represent the faces of an octahedral grain, and {110} crystal faces represent the faces of a rhombic dodecahedral grain, such faces may also be found in more irregularly shaped grains. An example of grains having a different morphology and yet also having {100} crystalline faces are the {100} silver bromide tabular grains of U.S. Pat. No. 4,386,156 or the silver chloride tabular grains of U.S. Pat. Nos. 5,264,337, 5,275,930, and 5,292,632, and published European Patent Application 0 534 395. The grains of these references are tabular rather than cubic, and their primary faces have {100} Miller indices.
U.S. Pat. No. 5,275,930 similarly discloses {100} tabular grains. Specifically, it relates to chemically sensitized high chloride tabular grain emulsions comprising tabular grains having {100} major crystal faces. Epitaxies, which are deposits of silver halide on a host grain wherein the deposit is of a different halide composition then the host, and wherein there is minimal or no gradient of halide from the host to the deposit, are grown at one or more of the corners of the tabular grains.
Other examples of tabular grains having epitaxial growths on their surfaces are disclosed in U.S. Pat. No. 4,435,501.
European Patent Application 0 569 971, though not directed at tabular grains having epitaxial growths, discloses rectangular parallelogram shaped tabular grains having a {100} plane as the main plane, and having from one to four corners non-equivalently eliminated. Although the resulting corner faces may be of the {111} type, they are formed from eliminated corners rather than by protruding silver halide. Thus, the grains' major surfaces maintain an essentially planar form. Further, the projections of these grains are either rectangular parallelograms having one corner eliminated, or rectangular parallelograms having multiple corners eliminated but wherein the eliminated corners are non-equivalent; that is, the area of the maximum eliminated portion divided by the area of the minimum eliminated portion is two or more.
Not all tabular grains need to be bounded by {100} major faces. U.S. Pat. No. 4,968,595, for instance, discloses tabular grains having no epitaxial junctions, and having {111} major faces which initiate development at the corners or edges of the grains.
European Patent Application 0 460 656, discloses tabular silver halide grains that have {111} major faces. Dislocations reside within the major surface regions of such grains. Similarly, U.S. Pat. No. 4,643,966 discloses a silver halide emulsion containing host tabular grains having {111} major faces. However, the major faces of these grains are subsequently ruffled by the precipitation or ripening of additional silver halide onto the major faces. Ruffles are protrusions emanating from the tabular surface of the host grains. The protrusions may be small three sided "pyrimids", each side of the pyramid having other than a {111} crystal face.
European Patent Application 0 302 528 does not disclose tabular grains as described above but does disclose silver halide grains comprising both a {111} plane and a {100} plane and capable of forming a latent image on the {100} plane. The {111} plane occupies at least 40% of the surface of the grains or the {100} plane occupies at least 60% of the surface of the grains.
U.S. Pat. No. 4,977,074 discloses substantially circular tabular grains having both a twin plane and a basal plane. The basal planes are of the {111} type and the grains have a {100} plane at their rounded edge portions.
Although the art is replete with examples of tabular grains, of tabular grains having distortions on their surfaces, and of tabular grains having epitaxial deposits on their surfaces, the art has yet to provide tabular {100} grains having multiple corner or side faces wherein the corner or side faces are equivalent and are selected to be of a different type than the major faces of the {100} tabular grains. It has further been unrecognized in the art how to grow protruding {111} or {110} corner or side faces on {100} tabular grains wherein the growth of the non-{100} corner or side faces on {100} tabular grains is accomplished in the absence of epitaxial deposition.